The colour temperature is adjusted in display devices in order that the picture can be seen subjectively in the same way in different lighting conditions. The natural light of the sun has a low colour temperature, about 5000 K. In artificial lighting, a filament lamp has a low colour temperature, but a fluorescent lamp has a high colour temperature, about 9000 K. Thus the colour temperature of the lighting of the environment can vary greatly, in the range 5000 to 10000 K. White material reflects all the wavelengths of light, and thus the lighting of the environment has the strongest effect on the observed colour temperature reflected by white material. With other colours this effect is smaller. Because of the difference of the colour temperatures of different lighting, a material seen as white in sunlight is seen as bluish in fluorescent lighting, for example.
When it is desired that the colours shown on the display screen, particularly white, are seen similarly as a material of corresponding colour beside the display is seen, the colour temperature of the display device must be adjusted to correspond to the lighting.
In a prior art solution, the adjustment of the colour temperature is carried out by changing the amount of blue and red colour in the white colour shown on the display. The white colour is shown on the display as a combination of red, green and blue.
The operation of the known liquid crystal displays is usually based on adjusting the amount of background light allowed to pass by means of a liquid crystal element. When the purpose is to show a fully lit bright point, the light is let through the element as well as possible. When the purpose is to show a black point, the light is prevented from passing through the element. Unlike the phosphorus surface of a cathode-ray tube or a light emitting diode (LED), the liquid crystal element does not produce light itself, but needs a light source for displaying a bright point. The environment and/or separate background lighting devices function as this source of light. The light of the environment can be led from the back to the liquid crystal element or from the front through the liquid crystal element and then reflected back with a mirror. The background lighting is created with lamps and reflected on the full picture area of the liquid crystal element with background light panels. Background lighting is particularly advantageously used in monitors, because the light of the environment alone is not sufficient to produce a picture with sufficient brightness.
FIG. 1 shows the mechanical principle of a prior art back-lit liquid crystal panel. The backlight lamps 1a, 1b, 1c, 1d, of which there are preferably 2 or 4, light through reflectors 2a, 2b the background light panel 3. The background light panel 3 contains a background light conductor, which divides the light over an area corresponding to the whole display screen on the liquid crystal element 4. In the example of FIG. 1, the bright items of element 4 starting from item 4a let light through, but the crossed items starting from item 4b do not. In the colour filter 5, the red, green and blue filter items which let through only light that is the colour of the filter item correspond to the items of element 4. The liquid crystal element 4 and the colour filter 5 have in practice been implemented as a solid component, which is shown by the line 6. A white point is formed on the display by conducting the background light with the liquid crystal element 4 to the red, green and blue filter items corresponding to the point. This is done by brightening the items of the liquid crystal element 4--three items of different colours for one point--which are all brightened. Red, green or blue and mixtures of these colours are formed by conducting the background light with the liquid crystal element 4 to the filter items corresponding to these colours. The light intensity of the liquid crystal panel is adjusted by changing the brightness of the background light lamps 1a, 1b, 1c, 1d.
The colour temperature is adjusted in the prior art liquid crystal panels by changing the pass rate of the background light by means of a liquid crystal element 4. When the purpose is to have a high colour temperature, such as 9300 K, which creates a bluish image, the passing of the red light is prevented more than the passing of the blue light. An adjustment of this type is here called passive adjustment, because the adjustment is based on preventing the passing of the component of light through the panel.
The prior art devices have the problem that when light is produced with background lamps, which have their own colour temperatures, like filament lamps and fluorescent tubes, it is necessary to prevent a very large amount of a colour which is not needed much for the desired colour temperature in question. As a result of this, the total light intensity is reduced substantially, and the picture gets dimmer. The colour temperature of a background light lamp is generally about 5000 to 6000 K, but the desired colour temperature of the picture can be, for example, 9000 K, and therefore a large part of the light must be prevented. Because of this, the adjustment range of liquid crystal panels is narrow.